1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor. In addition, the present invention also relates to an image forming method, an image forming apparatus, and a process cartridge using the electrophotographic photoreceptor.
2. Discussion of the Related Art
Electrophotographic image processing system devices have drastically improved recently. In particular, laser printers and digital copiers, which convert information into digital signals and record it optically, have drastically improved their printing quality and reliability. These laser printers and digital copiers have been merged with high-speed printing technologies and applied to full-color printing technologies recently. Because of this situation, it is desirable that electrophotographic photoreceptors (hereinafter simply “photoreceptors”) are capable of producing high quality images and highly durable.
Electrophotographic laser printers and digital copiers generally use organic photoreceptors, which comprise organic photosensitive materials, because of their low cost, high productivity, and nonpolluting property. Organic photoreceptors are broadly classified into single-layer photoreceptors and functionally-separated multilayer photoreceptors. The first organic photoreceptor having been put into practical use is a PVK-TNF charge-transfer-complex-type photoreceptor, which is one of the single-layer photoreceptors. In 1968, each of Hayashi and Regensburger independently invented a PVK/a-Se multilayer photoreceptor. In 1977 and 1978, Meltz and Schlosser, respectively, have invented a multilayer photoreceptor in which the photosensitive layer is comprised of organic materials only. More specifically, the photosensitive layer comprises an organic pigment dispersing layer and an organic low-molecular-weight polymer dispersing layer. The former is what is called a charge generation layer (CGL) that absorbs light to generate charge. The latter is what is called a charge transport layer (CTL) that injects and transports the charge generated in the CGL to neutralize surface charge. Such a multilayer photoreceptor may be called a functionally-separated multilayer photoreceptor. Functionally-separated multilayer photoreceptors have made drastic improvement in sensitivity and durability compared to single-layer photoreceptors. CGL and CTL include a charge generation material (CGM) and a charge transport material (CTM), respectively, which have different functions. Since CGM and CTM can be independently molecular-designed, CGM and CTM have wide ranges of choice for usable materials. For these reasons, functionally-separated multilayer photoreceptors have become the mainstream of organic photoreceptors.
A mechanism of forming electrostatic latent images in functionally-separated photoreceptors is considered as follows. A photoreceptor which has been charged is exposed to light. The light is transmitted by a charge transport layer and is absorbed by a charge generation material in a charge generation layer to generate charge. The charge generated in the charge generation layer is injected into the charge transport layer at the interface between the charge generation layer and the charge transport layer. The charge migrates through the charge transport layer due to an electric field to neutralize surface charge of the photoreceptor. As a result, an electrostatic latent image is formed on the photoreceptor.
Disadvantageously, photosensitive layers of organic photoreceptors are likely to be abraded in repeated use. Abrasion of photosensitive layers may accelerate deterioration of charged potential and photosensitivity of photoreceptors and the resultant, image density and quality. Therefore, organic photoreceptors have been improved to have better abrasion resistance. In accordance with recent speeding-up and downsizing of electrophotographic apparatuses, photoreceptors have been also downsized, i.e., the diameter of photoreceptors has been reduced. For this reason, organic photoreceptors have been improved to have much better abrasion resistance lately.
In order to improve abrasion resistance of photoreceptors, various approaches have been proposed. For example, lubricating or hardening photosensitive layers, including a filler in photosensitive layers, or using charge transport polymers instead of polymers in which low-molecular-weight charge transport materials are dispersed. However, even when abrasion of photosensitive layer is prevented by the above approaches, other problems may occur. For example, oxidizing substances such as ozone and NOx, which are produced in repeated use of photoreceptors depending on surrounding environmental conditions, may adsorb to the surface of photosensitive layers and reduce electric resistance thereof, causing image blurring. Conventional photoreceptors have avoided such a problem because oxidizing substances can be removed along with abrasion of photosensitive layers. By contrast, recent highly-durable photoreceptors cannot avoid the problem in the same way because photosensitive layers have been improved to have high abrasion resistance, as described above. In view of this situation, one proposed approach includes providing a heater to photoreceptors so that oxidizing substances are vaporized. This approach is against the recent trends to downsize apparatuses and to reduce electric power consumption. Another proposed approach includes including an antioxidant in photosensitive layers. Since typical antioxidants have no photoconductivity, this approach may cause deterioration of sensitivity and increase of residual potential of photoreceptors when the amount of antioxidants in photosensitive layer is too large.
Accordingly; highly-abrasion-resistant photoreceptors may produce side effects such as the occurrence of image blurring and deterioration of image resolution. It may be difficult for photoreceptors to have high durability and to produce high quality images simultaneously. In order to prevent image blurring, electric resistance is preferably as large as possible. By contrast, in order to prevent increase of residual potential, electric resistance is preferably as small as possible. Such a tradeoff makes it more difficult to realize high durability and high quality image simultaneously.
Japanese Patent Application Publication No. (hereinafter JP-A) 2000-231204 discloses an aromatic compound having a dialkylamino group as an acid scavenger. It is disclosed therein that the aromatic compound prevents the occurrence of image blurring which is caused by oxidizing gases even after a photoreceptor is repeatedly used. However, the aromatic compound has too low charge transport ability to respond to demands of highly-sensitive and high-speed photoreceptors.
JP-A 60-196768 and Japanese Patent No. 2884353 each disclose stilbene compounds having a dialkylamino group. A technical document “The Effects of Nitrogen Oxide on the Resolution of Organic Photoconductors (Itami et al, Konica technical Report Vol. 13 (2000) p. 37-40)” reports that the above stilbene compounds prevent the occurrence of image blurring which is caused by oxidizing gases. The stilbene compounds have a triarylamine structure, which serves as a charge transporting site. The triarylamine structure has a dialkylamino group, which is a substituent having a strong mesomeric effect (i.e., +M effect) on a resonance position. Therefore, the ionized potential of the stilbene compound is extremely small. In a case in which such a stilbene compound is used alone as a charge transport material in a photosensitive layer, the charge retention capability of the photosensitive layer may be extremely poor from the initial stage, or may degrade with time. For this reason, it is difficult to put the stilbene compounds into practical use. Even when the stilbene compound is used in combination with another charge transport material, the stilbene compound may disadvantageously serve as a hole trapping site because the ionized potential of the stilbene compound is considerably smaller than that of the other charge transport material. As a result, the resultant photoreceptor may have extremely low sensitivity and high residual potential.